Transistors with Stacked Semiconductor Layers as Channels

ABSTRACT

A method of forming a semiconductor device includes depositing a p-type semiconductor layer over a portion of a semiconductor substrate, depositing a semiconductor layer over the p-type semiconductor layer, wherein the semiconductor layer is free from p-type impurities, forming a gate stack directly over a first portion of the semiconductor layer, and etching a second portion of the semiconductor layer to form a trench extending into the semiconductor layer. At least a surface of the p-type semiconductor layer is exposed to the trench. A source/drain region is formed in the trench. The source/drain region is of n-type.

PRIORITY CLAIM AND CROSS-REFERENCE

This application claims the benefit of the following provisionally filedU.S. Patent application: Application Ser. No. 62/751,094, filed Oct. 26,2018, and entitled “Transistors with Stacked Semiconductor Layers asChannels,” which application is hereby incorporated herein by reference.

BACKGROUND

With the advancement of the integrated circuits, the density of theintegrated circuit devices such as transistors is becoming increasinglyhigher, and the devices are becoming increasingly smaller. This providesa more demanding requirement to the performance of the integratedcircuit devices. For example, the leakage currents need to be smaller,and the drive currents need to be higher.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1-3, 4A, 4B, 5A, 5B, 6A, 6B, 7A, 7B, 8, 9, 10A, 10B, 11, 12A, and12B illustrate the perspective views and cross-sectional views in theformation of transistors in accordance with some embodiments.

FIG. 13 illustrates a process flow for forming a transistor inaccordance with some embodiments.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

A transistor and the method of forming the same are provided inaccordance with various embodiments. The intermediate stages in theformation of the transistor are illustrated in accordance with someembodiments. Some variations of some embodiments are discussed.Throughout the various views and illustrative embodiments, likereference numbers are used to designate like elements. In accordancewith some embodiments of the present disclosure, a transistor includestacked silicon layer(s) and p-type semiconductor layers (such assilicon boron (SiB) layer(s)), which are used to form the channel regionof the corresponding transistor, so that the leakage between sourceregion and drain region is reduced. It is appreciated that the formationof a Fin Field-Effect Transistor is used as an example to explain theconcept of the present disclosure. The embodiments of the presentdisclosure are readily applicable to other types of transistors such asplanar transistors, Gate-All-Around (GAA) transistors, and the like.Also, it is appreciated that although n-type transistors are discussedin the examples of the embodiments, p-type transistors may also beformed by applying the concepts of the present disclosure. The p-typetransistors may be similar to the n-type transistors, except that thep-type semiconductor layers in the stacked semiconductor layers of then-type transistors are replaced with n-type semiconductor layers, thep-well region is replaced with an n-well region, and n-type source/drainregions are replaced with p-type source/drain regions.

FIGS. 1 through 3, 4A, 4B, 5A, 5B, 6A, 6B, 7A, 7B, 8, 9, 10A, 10B, 11,12A, and 12B illustrate the perspective views and cross-sectional viewsin the formation of n-type transistors in accordance with someembodiments. The corresponding processes are also reflectedschematically in the process flow 200 as shown in FIG. 13.

In FIG. 1, substrate 20 is provided. The substrate 20 may be asemiconductor substrate, such as a bulk semiconductor substrate, aSemiconductor-On-Insulator (SOI) substrate, or the like, which may bedoped (e.g., with a p-type or an n-type dopant) or undoped. Thesemiconductor substrate 20 may be a part of wafer 10, such as a siliconwafer. Generally, an SOI substrate is a layer of a semiconductormaterial formed on an insulator layer. The insulator layer may be, forexample, a Buried Oxide (BOX) layer, a silicon oxide layer, or the like.The insulator layer is provided on a substrate, typically a silicon orglass substrate. Other substrates, such as a multi-layered or gradientsubstrate may also be used. In some embodiments, the semiconductormaterial of semiconductor substrate 20 may include silicon; germanium; acompound semiconductor including silicon carbide, gallium arsenic,gallium phosphide, indium phosphide, indium arsenide, and/or indiumantimonide; an alloy semiconductor including SiGe, GaAsP, AlInAs,AlGaAs, GaInAs, GaInP, and/or GaInAsP; or combinations thereof.

Further referring to FIG. 1, well region 22 is formed in substrate 20.The respective process is illustrated as process 202 in the process flow200 shown in FIG. 25. In accordance with some embodiments of the presentdisclosure, well region 22 is a p-type well region formed throughimplanting a p-type impurity, which may be boron, indium, or the like.The resulting well region 22 may extend to the top surface of substrate20. The p-type impurity concentration may be equal to or less than 10¹⁸cm⁻³, such as in the range between about 10¹⁷ cm⁻³and about 10¹⁸ cm⁻³.

Referring to FIG. 2, isolation regions 24 are formed to extend from atop surface of substrate 20 into substrate 20. Isolation regions 24 arealternatively referred to as Shallow Trench Isolation (STI) regionshereinafter. The respective process is illustrated as process 204 in theprocess flow shown in FIG. 13. The portions of substrate 20 betweenneighboring STI regions 24 are referred to as semiconductor strips 26.To form STI regions 24, pad oxide layer 28 and hard mask layer 30 areformed on semiconductor substrate 20, and are then patterned. Pad oxidelayer 28 may be a thin film including silicon oxide. In accordance withsome embodiments of the present disclosure, pad oxide layer 28 is formedin a thermal oxidation process, wherein a top surface layer ofsemiconductor substrate 20 is oxidized. Pad oxide layer 28 acts as anadhesion layer between semiconductor substrate 20 and hard mask layer30. Pad oxide layer 28 may also act as an etch stop layer for etchinghard mask layer 30. In accordance with some embodiments of the presentdisclosure, hard mask layer 30 is formed of silicon nitride, forexample, using Low-Pressure Chemical Vapor Deposition (LPCVD). Inaccordance with other embodiments of the present disclosure, hard masklayer 30 is formed by thermal nitridation of silicon, or Plasma EnhancedChemical Vapor Deposition (PECVD). A photo resist (not shown) is formedon hard mask layer 30 and is then patterned. Hard mask layer 30 is thenpatterned using the patterned photo resist as an etching mask to formhard masks 30 as shown in FIG. 2.

Next, the patterned hard mask layer 30 is used as an etching mask toetch pad oxide layer 28 and substrate 20, followed by filling theresulting trenches in substrate 20 with a dielectric material(s). Aplanarization process such as a Chemical Mechanical Polish (CMP) processor a mechanical grinding process is performed to remove excessingportions of the dielectric materials, and the remaining portions of thedielectric materials(s) are STI regions 24. STI regions 24 may include aliner dielectric (not shown), which may be a thermal oxide formedthrough thermal oxidation of a surface layer of substrate 20. The linerdielectric may also be a deposited silicon oxide layer, silicon nitridelayer, or the like formed using, for example, Atomic Layer Deposition(ALD), High-Density Plasma Chemical Vapor Deposition (HDPCVD), orChemical Vapor Deposition (CVD). STI regions 24 may also include adielectric material over the liner oxide, wherein the dielectricmaterial may be formed using Flowable Chemical Vapor Deposition (FCVD),spin-on coating, or the like. The dielectric material over the linerdielectric may include silicon oxide in accordance with someembodiments.

In accordance with some embodiments of the present disclosure, thebottoms of well region 22 is lower than the bottom surfaces of STIregions 24, and hence semiconductor strips 26 are parts of well region22, and are doped with the p-type impurity for forming well region 22.

In a subsequent process, pad oxide layer 28 and hard mask layer 30 areremoved. Next, as shown in FIG. 3, semiconductor strips 26 are recessed,so that trenches 32 are formed between neighboring STI regions 24. Therespective process is illustrated as process 206 in the process flowshown in FIG. 13. In accordance with some embodiments of the presentdisclosure, the recessing is performed through dry etch. The dry etchmay be performed using an etching gas selected from C₂F₆, CF₄, SO₂, themixture of HBr, Cl₂, and O₂, the mixture of HBr, Cl₂, and O₂, or themixture of HBr, Cl ₂, O₂, and CF₂ etc., or the like. In accordance withalternative embodiments, the etching is performed using a wet etchingmethod, in which KOH, tetramethylammonium hydroxide (TMAH), CH₃COOH,NH₄OH, H₂O₂, Isopropanol (IPA), the solution of HF, HNO₃, and H₂O, orthe like, is used as the etchant. In accordance with some embodiments,the bottoms of trenches 32 are higher than the bottom surfaces of STIregions 24.

FIG. 4A illustrates the formation of stacked semiconductor layers 34(with the details shown in FIG. 4B), which are formed through SelectiveEpitaxial Growth (SEG). The stacked semiconductor layers 34 are formedin trenches 32 as shown in FIG. 3. The respective process is illustratedas process 208 in the process flow shown in FIG. 13. In accordance withsome embodiments of the present disclosure, stacked semiconductor layers34 include a plurality of stacked layers including at least two, andpossibly more, silicon layers and at least one, and possibly more,p-type epitaxy layers (such as SiB layers), which are discussed indetail referring to FIG. 4B. The epitaxially grown semiconductor layersmay be grown to a level higher than the top surfaces of STI regions 24.In a subsequent process, a planarization process such as a CMP processor a mechanical grinding process is performed to remove excess portionsof the grown semiconductor materials, resulting in the structure shownin FIGS. 4A and 4B.

FIG. 4B illustrates the reference cross-section 4B-4B in FIG. 4A, exceptthe details of stacked semiconductor layers 34 are illustrated. Inaccordance with some embodiments of the present disclosure, siliconlayer 34A is epitaxially grown first. The thickness of silicon layer 34Amay be in the range between about 1 nm and about 5 nm. In accordancewith some embodiments of the present disclosure, silicon layer 34A is anintrinsic layer that is neither intentionally doped with any p-typeimpurity nor intentionally doped with any n-type impurity. In accordancewith alternative embodiments of the present disclosure, silicon layer34A is doped with a p-type impurity such as boron, indium, or the like,with an impurity concentration lower than the p-type impurityconcentration of the overlying p-type semiconductor layer 34B by atleast one order, two orders, or more. Accordingly, silicon layer 34A, ifdoped with a p-type impurity, may have an impurity concentration lowerthan about 10¹⁷ cm⁻³, or lower than about 10¹⁶ cm⁻³, or lower. Inaccordance with other embodiments, layer 34A may be formed of othersemiconductor materials such as silicon germanium, silicon carbon, orthe like, which may be intrinsic layers undoped with (or lightly dopedwith) any p-type or n-type impurities.

P-type epitaxy semiconductor layer 34B is epitaxially grown on siliconlayer 34A. In accordance with some embodiments of the presentdisclosure, p-type semiconductor layer 34B comprises silicon and ap-type impurity such as boron, indium, or the like. For example, p-typesemiconductor layer 34B may be a silicon boron (SiB) layer. The p-typeimpurity is in-situ doped with the proceeding of the epitaxy of p-typesemiconductor layer 34B. The p-type impurity concentration in p-typesemiconductor layer 34B cannot be too high since this may cause thep-type impurity to be undesirably diffused into the underlying siliconlayer 34A and the overlying silicon layer 34C, which causes the leakageprevention ability to be undesirably compromised. For example, thep-type impurity concentration in p-type semiconductor layer 34B may belower than about 5×10²⁰ cm⁻³, or lower than about 1×10¹⁹ cm⁻³. Thep-type impurity concentration in p-type semiconductor layer 34B alsocannot be too low since the p-type impurity in in p-type semiconductorlayer 34B generate holes, and if the p-type impurity concentration istoo low, the number of the generated holes is too low, which againcauses the leakage prevention ability to be undesirably compromised. Forexample, the p-type impurity concentration in p-type semiconductor layer34B may be in the range between about 5×10¹⁷ cm⁻³ and about 5×10²⁰ cm⁻³,and may be in the range between about 1×10¹⁸ cm⁻³ and about 1×10¹⁹ cm⁻³.In accordance with some embodiments, p-type semiconductor layer 34B isfree from germanium, carbon, or the like. In accordance with alternativeembodiments, p-type semiconductor layer 34B includes silicon and anelement selected from germanium, carbon, or the like. The thickness ofp-type semiconductor layer 34B may be in the range between about 1 nmand about 15 nm.

Over p-type semiconductor layer 34B, another silicon layer 34C isepitaxially grown. In accordance with some embodiments of the presentdisclosure, silicon layer 34C is an intrinsic layer that is neitherintentionally doped with any p-type impurity nor intentionally dopedwith any n-type impurity. In accordance with alternative embodiments ofthe present disclosure, silicon layer 34C is doped with a p-typeimpurity such as boron, indium, or the like, with an impurityconcentration lower than the p-type impurity concentration of theunderlying p-type epitaxy semiconductor layer 34B by at least one order,two orders, or more. Accordingly, silicon layer 34C, if doped with ap-type impurity, may have an impurity concentration lower than about10¹⁷ cm⁻³, or lower than about 10¹⁶ cm⁻³, or lower. Depending on whetherthere are additional epitaxy semiconductors layers 34D and 34E formedover silicon layer 34C or not, the thickness of silicon layer 34C may bein a large range between about 14 nm and about 51 nm.

In accordance with some embodiments of the present disclosure, theepitaxy process is finished after the formation of silicon layer 34C,and no additional semiconductor layer is epitaxially grown over siliconlayer 34C. In accordance with alternative embodiments of the presentdisclosure, p-type epitaxy semiconductor layer 34D is further grown oversilicon layer 34C, and no additional semiconductor layer is epitaxiallygrown over p-type epitaxy semiconductor layer 34D. In accordance withyet alternative embodiments of the present disclosure, p-type epitaxysemiconductor layer 34D is grown over silicon layer 34C, and siliconlayer 34E is further grown over p-type epitaxy semiconductor layer 34D.Accordingly, p-type epitaxy semiconductor layer 34D and silicon layer34E are illustrated using dashed lines to indicate that they may be, ormay not be, formed.

P-type epitaxy semiconductor layer 34D (if formed) is epitaxially grownon silicon layer 34C. In accordance with some embodiments of the presentdisclosure, p-type epitaxy semiconductor layer 34D comprises silicon anda p-type impurity such as boron, indium, or the like. For example,p-type epitaxy semiconductor layer 34D may be a SiB layer. The p-typeimpurity is in-situ doped with the proceeding of the epitaxy of p-typeepitaxy semiconductor layer 34D. Similarly, the p-type impurityconcentration in p-type semiconductor layer 34B cannot be too high ortoo low. Otherwise, the electron-hole combining function of the p-typeepitaxy semiconductor layer is compromised. In accordance with someembodiments of the present disclosure, the p-type impurity concentrationin p-type epitaxy semiconductor layer 34D is in the range between about5×10¹⁷ cm⁻³ and about 5×10²⁰ cm⁻³, and may be in the range between about1×10¹⁸ cm⁻³ and about 1×10¹⁹ cm⁻³. In accordance with some embodiments,p-type semiconductor layer 34B is free from germanium, carbon, or thelike. The thickness of p-type epitaxy semiconductor layer 34D may be inthe range between about 1 nm and about 15 nm.

Over p-type epitaxy semiconductor layer 34D, another silicon layer 34Emay be epitaxially grown, or the formation of silicon layer 34E may beskipped. In accordance with some embodiments of the present disclosure,silicon layer 34E is an intrinsic layer that is neither intentionallydoped with any p-type impurity nor intentionally doped with any n-typeimpurity. In accordance with alternative embodiments of the presentdisclosure, silicon layer 34E is doped with a p-type impurity such asboron, indium, or the like, with an impurity concentration lower thanthe p-type impurity concentration of the underlying p-type epitaxysemiconductor layers 34B and 34D by at least one order, two orders, ormore. Accordingly, silicon layer 34E, if doped with a p-type impurity,may have an impurity concentration lower than about 10¹⁷ cm⁻³, or lowerthan about 10¹⁶ cm⁻³, or lower. Silicon layer 34E (if formed) may beused as a buffer layer to receive the planarization process (FIG. 4A),and to protect the underlying p-type epitaxy semiconductor layer 34Dfrom receiving the planarization. The thickness of silicon layer 34E maybe small, and may be controlled to be as small as possible, as long asit can protect p-type epitaxy semiconductor layer 34D from beingplanarized with adequate process margin. In accordance with someembodiments of the present disclosure, the thickness of silicon layer34E is in the range between about 1 nm and about 5 nm.

Next, referring to FIG. 5A, STI regions 24 are recessed such that atleast the upper portions of stacked semiconductor layers 34 protrudehigher than the top surfaces of neighboring STI regions 24. Therespective process is illustrated as process 210 in the process flowshown in FIG. 13. Furthermore, STI regions 24 may have a flat surface asillustrated, a convex top surface, a concave top surface (such asdishing), or a combination thereof. The top surfaces of the STI regions24 may be formed flat, convex, and/or concave by an appropriate etch.The STI regions 24 may be recessed using an acceptable etching processusing an etchant that attacks STI regions 24, but does not attacksemiconductor layers 34. For example, if wet etch is used, the etchantmay include dilute hydrofluoric (dHF) acid. If dry etch is used, amixture of NF₃ and NH₃ gases or a mixture of HF and NH₃ gases may beused. The portions of semiconductor material higher than the topsurfaces of STI regions 24 are referred to as protruding fins 36.

FIG. 5B illustrates the reference cross-section 5B-5B in FIG. 5A, exceptthe details of stacked semiconductor layers 34 are illustrated. SinceSTI regions 24 are not in the illustrated plane, STI regions 24 are notshown in FIG. 5B. The levels of the top surfaces 24A and bottom surfaces24B of STI regions 24 are illustrated to show the level of STI regions24. In accordance with some embodiments of the present disclosure, thetop surfaces 24A of STI regions 24 are at an intermediate level betweenthe top surface and the bottom surface of silicon layer 34A. Inaccordance with alternative embodiments, the top surfaces 24A of STIregions 24 are level with the top surface of silicon layer 34A. The topsurfaces 24A of STI regions 24 may also be level with or lower than thebottom surface of silicon layer 34A.

In above-illustrated embodiments, the fins may be patterned by anysuitable method. For example, the fins may be patterned using one ormore photolithography processes, including double-patterning ormulti-patterning processes. Generally, double-patterning ormulti-patterning processes combine photolithography and self-alignedprocesses, allowing patterns to be created that have, for example,pitches smaller than what is otherwise obtainable using a single, directphotolithography process. For example, in one embodiment, a sacrificiallayer is formed over a substrate and patterned using a photolithographyprocess. Spacers are formed alongside the patterned sacrificial layerusing a self-aligned process. The sacrificial layer is then removed, andthe remaining spacers, or mandrels, may then be used to pattern thefins.

Referring to FIG. 6A, dummy gate stacks 38 are formed to extend on thetop surfaces and the sidewalls of (protruding) fins 36. The respectiveprocess is illustrated as process 212 in the process flow shown in FIG.13. Dummy gate stacks 38 may include dummy gate dielectrics 40 and dummygate electrodes 42 over dummy gate dielectrics 40. Dummy gate electrodes42 may be formed, for example, using polysilicon, and other materialsmay also be used. Each of dummy gate stacks 38 may also include one (ora plurality of) hard mask layer 44 over dummy gate electrodes 42. Hardmask layers 44 may be formed of silicon nitride, silicon oxide, siliconcarbo-nitride, or multi-layers thereof. Dummy gate stacks 38 may crossover a single one or a plurality of protruding fins 36 and/or STIregions 24. Dummy gate stacks 38 also have lengthwise directionsperpendicular to the lengthwise directions of protruding fins 36.

Next, gate spacers 46 are formed on the sidewalls of dummy gate stacks38. The respective process is also illustrated as process 212 in theprocess flow shown in FIG. 13. In accordance with some embodiments ofthe present disclosure, gate spacers 46 are formed of a dielectricmaterial(s) such as silicon nitride, silicon carbo-nitride, or the like,and may have a single-layer structure or a multi-layer structureincluding a plurality of dielectric layers. FIG. 6B illustrates thereference cross-section 6B-6B in FIG. 6A. It is appreciated that sincelayers 34D and 34E may or may not be formed, gate stack 38 may have abottom surface contacting the top surface of silicon layer 34E, p-typeepitaxy semiconductor layer 34D, or silicon layer 34C.

An etching step is then performed to recess the portions of stackedsemiconductor layers 34 that are not covered by dummy gate stack 38 andgate spacers 46, resulting in the structure shown in FIG. 7A. Therespective process is illustrated as process 214 in the process flowshown in FIG. 13. The recessing may be anisotropic, and hence theportions of fins 36 directly underlying dummy gate stacks 38 and gatespacers 46 are protected, and are not etched. The top surfaces of therecessed stacked semiconductor layers 34 may be lower than the topsurfaces 24A of STI regions 24 in accordance with some embodiments.Recesses 50 are accordingly formed. Recesses 50 comprise portionslocated on the opposite sides of dummy gate stacks 38, and portionsbetween remaining portions of protruding fins 36.

FIG. 7B illustrates the reference cross-section 7B-7B in FIG. 7A. Inaccordance with some embodiments of the present disclosure, the bottomsof recesses 50 are at the bottom surface level of p-type semiconductorlayer 34B, and hence recesses 50 penetrate through p-type semiconductorlayer 34B. The sidewalls of the remaining portions of p-typesemiconductor layer 34B are exposed to recesses 50. In accordance withalternative embodiments, the bottoms of recesses 50 are at the topsurface level of p-type epitaxy semiconductor layer 34B, and the topsurface of p-type epitaxy semiconductor layer 34B are exposed. Inaccordance with yet alternative embodiments, the bottoms of recesses 50are at a level between the top surface level and the bottom surfacelevel of p-type epitaxy semiconductor layer 34B. Also, the bottomsurfaces of recesses 50 may be at a level between the top surfaces andthe bottom surfaces of STI regions 24. The bottom surfaces of recesses50 may also be higher than or lower than the top surface of STI regions24. Dashed lines 52 illustrate the likely positions of the bottomsurfaces of recesses 50. It is preferred that recesses 50 does notpenetrate through silicon layer 34A, so that the implanted well region22 is not exposed to recesses 50, and the subsequently formedsource/drain regions 54 (FIG. 8) is spaced apart the implanted wellregion 22, which has more defects than the epitaxy semiconductor layers34 and hence may cause more junction leakage.

Next, an epitaxy process is performed to form epitaxy regions 54, whichare selectively grown from recesses 50, resulting in the structure inFIG. 8. The respective process is illustrated as process 216 in theprocess flow shown in FIG. 13. In accordance with some embodiments,epitaxy regions 54 include SiP, SiCP, SiC, or the like, which may have alattice constant smaller than that of silicon. In accordance with someembodiments of the present disclosure, an n-type impurity such asphosphorous, indium, antimony, or the like is in-situ doped into epitaxyregions 54 with the proceeding of the epitaxy. After epitaxy regions 54fully fill recesses 50, epitaxy regions 54 start expanding horizontally,and facets may be formed. The neighboring epitaxy regions 54 startmerging with each other. As a result, an integrated epitaxy region 54 isformed. The top surface of source/drain regions 54 may be higher thanthe bottom surfaces of gate spacers 46.

Voids (air gaps) 56 may be generated. In accordance with someembodiments of the present disclosure, the formation of epitaxy regions54 is finished when the top surfaces of epitaxy regions 54 are stillwavy (FIG. 8), or when the top surfaces of the merged epitaxy regions 54have become planar (FIG. 9), which is achieved by further growing on theepitaxy regions 54 as shown in FIG. 8. After the formation of epitaxyregions 54, an implantation process may be performed to implant ann-type impurity into epitaxy regions 54, forming source/drain regions,which are also denoted as source/drain regions 54. In accordance withalternative embodiments in which an n-type impurity has been in-situincorporated, the implantation process is skipped.

FIG. 10A illustrates a perspective view of the structure after theformation of Contact Etch Stop Layer (CESL) 58 and Inter-LayerDielectric (ILD) 60. The respective process is illustrated as process218 in the process flow shown in FIG. 13. CESL 58 may be formed ofsilicon oxide, silicon nitride, silicon carbo-nitride, or the like, andmay be formed using CVD, ALD, or the like. ILD 60 may include adielectric material formed using, for example, FCVD, spin-on coating,CVD, or another deposition method. ILD 60 may be formed of anoxygen-containing dielectric material, which may be a silicon-oxidebased material such as Tetra Ethyl Ortho Silicate (TEOS) oxide,Plasma-Enhanced CVD (PECVD) oxide (SiO₂), Phospho-Silicate Glass (PSG),Boro-Silicate Glass (BSG), Boron-Doped Phospho-Silicate Glass (BPSG), orthe like. A planarization process such as a CMP process or a mechanicalgrinding process may be performed to level the top surfaces of ILD 60,dummy gate stacks 38, and gate spacers 46 with each other.

FIG. 10B illustrates the reference cross-section 10B-10B in FIG. 10A. Asshown in FIG. 10B, source/drain regions 54 are at least in contact withp-type semiconductor layer 34B. For example, depending on whethersource/drain regions 54 penetrate through p-type semiconductor layer 34Bor not, source/drain regions 54 may be in contact with the top surfaceand/or the sidewall of p-type epitaxy semiconductor layer 34B.Source/drain regions 54 are also in contact with the sidewalls ofsilicon layer 34C, and may be in contact with the sidewalls of p-typeepitaxy semiconductor layer 34D and silicon layer 34E, if formed.Source/drain regions 54 may be in contact with, and may or may notextend into, silicon layer 34A. When source/drain regions 54 extend intosilicon layer 34A, source/drain regions 54 may not penetrate throughsilicon layer 34A.

In accordance with some embodiments, p-type semiconductor layer 34B isclose to the bottom of source/drain regions 54. For example, the depthD1 of the top surface of p-type semiconductor layer 34B may be greaterthan about 80 percent the depth D2 of source/drain regions 54, whereindepths D1 and D2 are measured from the bottom of gate spacers 46. RatioD1/D2 may be up to 100 percent, which means that the bottom surface ofsource/drain regions 54 are in contact with the top surface of p-typesemiconductor layer 34B. Allocating p-type epitaxy semiconductor layer34B close to the bottom of source/drain regions 54 has more effect inimproving Drain-Induced Barrier Lowering (DIBL) performance of therespective transistor than allocating p-type epitaxy semiconductor layer34B to a higher position.

Next, dummy gate stacks 38, which include hard mask layers 44, dummygate electrodes 42, and dummy gate dielectrics 40, are replaced withreplacement gate stacks 68 (FIG. 11), which include metal gates 66 andgate dielectrics 64. The respective process is illustrated as process220 in the process flow shown in FIG. 13. When forming replacement gatestacks 68, hard mask layers 44, dummy gate electrodes 42, and dummy gatedielectrics 40 as shown in FIGS. 10A and 10B are first removed in one ora plurality of etching steps, resulting in trenches/openings to beformed between gate spacers 46. The top surfaces and the sidewalls ofprotruding semiconductor fins 36 are exposed to the resulting trenches.

As revealed in FIG. 10B, after dummy gate stacks 38 are exposed, stackedsemiconductor layers 34 are exposed to the resulting trenches. In somecases, the removal of dummy gate stacks 38 may not stop on the topsurface of the top silicon layer (34E (if formed), or 34C if 34E and 34Dare not formed) well. If this occurs, the resulting recess insemiconductor layers 34 may laterally extend toward source/drain regions54, and there is a possibility the subsequently formed gate electrodes66 may be electrically shorted to source/drain regions 54, or have highleakage currents therebetween. This effect is referred to as metal gateextrusion, which may cause device failure. P-type epitaxy semiconductorlayer 34D, which is formed close to the top surface of stackedsemiconductor layers 34, may act as the etch stop layer if silicon layer34E is etched-through since the etching rate of p-type epitaxysemiconductor layer 34D is lower than the etching rate of silicon layer34E when an appropriate etchant is used.

After the removal of dummy gate stacks 38, (replacement) gate dielectriclayers 64 are formed, which extend into the trenches between gatespacers 46. In accordance with some embodiments of the presentdisclosure, each of gate dielectric layers 64 includes an InterfacialLayer (IL) as its lower part, which contacts the exposed surfaces of thecorresponding protruding fins 36. The IL may include an oxide layer suchas a silicon oxide layer, which is formed through the thermal oxidationof protruding fins 36, a chemical oxidation process, or a depositionprocess. Gate dielectric layer 64 may also include a high-k dielectriclayer formed over the IL. The high-k dielectric layer may include ahigh-k dielectric material such as hafnium oxide, lanthanum oxide,aluminum oxide, zirconium oxide, silicon nitride, or the like. Thedielectric constant (k-value) of the high-k dielectric material ishigher than 3.9, and may be higher than about 7.0. The high-k dielectriclayer is formed as a conformal layer, and extends on the sidewalls ofprotruding fins 36 and the sidewalls of gate spacers 46. In accordancewith some embodiments of the present disclosure, the high-k dielectriclayer is formed using ALD or CVD.

Referring further to FIG. 11, gate electrodes 66 are formed over gatedielectrics 64, Gate electrodes 66 include conductive sub-layers. Thesub-layers are not shown separately, while the sub-layers aredistinguishable from each other. The deposition of the sub-layers may beperformed using a conformal deposition method(s) such as ALD or CVD.

The stacked conductive layers may include a diffusion barrier layer andone (or more) work-function layer over the diffusion barrier layer. Thediffusion barrier layer may be formed of titanium nitride (TiN), whichmay (or may not) be doped with silicon. The work-function layer (markedschematically as 66A in FIG. 12B) determines the work function of thegate, and includes at least one layer, or a plurality of layers formedof different materials. For example, the work-function layer 66A mayinclude a titanium aluminum (TiAl) layer. After the deposition of thework-function layer(s), a barrier layer, which may be another TiN layer,is formed.

The deposited gate dielectric layers and conductive layers are formed asconformal layers extending into the trenches between gate spacers 46,and include some portions over ILD 60. Next, a metallic material isdeposited to fill the remaining trenches between gate spacers 46. Themetallic material may be formed of tungsten or cobalt, for example. In asubsequent step, a planarization step such as a CMP process or amechanical grinding process is performed, so that the portions of thegate dielectric layers, conductive sub-layers, and the metallic materialover ILD 60 are removed. As a result, metal gate electrodes 66 and gatedielectrics 64 are formed. Gate electrodes 66 and gate dielectrics 64are in combination referred to as replacement gate stacks 68. The topsurfaces of replacement gate stacks 68, gate spacers 46, CESL 58, andILD 60 may be substantially coplanar at this time.

FIG. 11 also illustrates the formation of hard masks 70 in accordancewith some embodiments. The formation of hard masks 70 may includeperforming an etching step to recess gate stacks 68, so that recessesare formed between gate spacers 46, filling the recesses with adielectric material, and then performing a planarization process such asa CMP process or a mechanical grinding process to remove excess portionsof the dielectric material. Hard masks 70 may be formed of siliconnitride, silicon oxynitride, silicon oxy-carbo-nitride, or the like.

FIG. 12A illustrates the formation of source/drain contact plugs 72. Therespective process is illustrated as process 222 in the process flowshown in FIG. 13. The formation of source/drain contact plugs 72includes etching ILD 60 to expose the underlying portions of CESL 58,and then etching the exposed portions of CESL 58 to reveal epitaxyregions 54. In a subsequent process, a metal layer (such as a Ti layer)is deposited and extending into the contact openings. A metal nitridecapping layer may be performed. An anneal process is then performed toreact the metal layer with the top portion of source/drain regions 54 toform silicide regions 74, as shown in FIGS. 12A and 12B. Next, eitherthe previously formed metal nitride layer is left as not removed, or thepreviously formed metal nitride layer is removed, followed by thedeposition of a new metal nitride layer (such as titanium nitridelayer). A filling metallic material such as tungsten, cobalt, or thelike, is then filled into the contact openings, followed by aplanarization to remove excess materials, resulting in source/draincontact plug 72. Accordingly, source/drain contact plug 72 includes theremaining portions of the metal layer, metal nitride layer, and thefilling metallic material. Gate contact plugs (not) shown) are alsoformed to penetrate through a portion of each of hard masks 70 tocontact gate electrodes 66. FinFETs 78, which may be connected inparallel as one FinFET, is thus formed.

FIG. 12B illustrates the reference cross-section 12B-12B in FIG. 12A. Asshown in FIG. 12B, gate stacks 68 are over the stacked semiconductorlayers 34, which act as the channels of FinFET 78. Currents may flow instacked semiconductor layers 34, and may flow in both p-type epitaxysemiconductor layers (34B/34D) and silicon layers (34A/34C/34E). Gatestacks 68 may be in contact with silicon layer 34C, p-type epitaxysemiconductor layer 34D, or silicon layer 34E, depending on whetherp-type epitaxy semiconductor layer 34D and silicon layer 34E are formedor not.

The embodiments of the present disclosure have some advantageousfeatures. By forming a p-type epitaxy semiconductor layer at a levelclose to the bottom level of source/drain regions, the leaked electronsleaking between source and drain regions can recombine with the holes ofthe p-type epitaxy semiconductor layer, so that the leakage is reduced,and the DIBL performance is improved. By forming a p-type epitaxysemiconductor layer at a level close to the top level of source/drainregions, the p-type epitaxy semiconductor layer may act as an etch stoplayer, and has the function of preventing metal gate extrusion. Theproduction yield is improved.

In accordance with some embodiments of the present disclosure, a methodof forming a semiconductor device comprises depositing a first p-typesemiconductor layer over a portion of a semiconductor substrate;depositing a first semiconductor layer over the first p-typesemiconductor layer, wherein the first semiconductor layer is free fromp-type impurities; forming a gate stack directly over a first portion ofthe first semiconductor layer; etching a second portion of the firstsemiconductor layer to form a trench extending into the firstsemiconductor layer, wherein at least a surface of the first p-typesemiconductor layer is exposed to the trench; and forming a source/drainregion in the trench, wherein the source/drain region is of n-type. Inan embodiment, the method further comprises depositing a secondsemiconductor layer over the portion of a semiconductor substrate,wherein the second semiconductor layer is further free from p-typeimpurities, and the second semiconductor layer is underlying and incontact with the first p-type semiconductor layer. In an embodiment, inthe etching, the first p-type semiconductor layer is furtheretched-through, and a top surface of the second semiconductor layer isexposed to the trench. In an embodiment, a bottom surface of the trenchis higher than a bottom surface of the second semiconductor layer. In anembodiment, the etching stops on a top surface of the first p-typesemiconductor layer. In an embodiment, the method further comprisesdepositing a second p-type semiconductor layer over the firstsemiconductor layer. In an embodiment, the method further comprisesdepositing a second semiconductor layer over the second p-typesemiconductor layer, wherein the second semiconductor layer is free fromp-type impurities. In an embodiment, the method further comprisesforming a gate electric over and contacting the second semiconductorlayer. In an embodiment, the method further comprises etching a dummygate stack over the second semiconductor layer, wherein the secondsemiconductor layer is etched-through, and the etching stops on a topsurface of the second p-type semiconductor layer. In an embodiment, thefirst p-type semiconductor layer is further free from n-type impurities.

In accordance with some embodiments of the present disclosure, a methodof forming a semiconductor device includes forming isolation regionsextending into a semiconductor substrate; etching to remove a portion ofthe semiconductor substrate between the isolation regions to form atrench; performing a first epitaxy to grow a first semiconductor layerin the trench, wherein the first semiconductor layer is free from p-typeimpurities and n-type impurities; performing a second epitaxy to grow afirst SiB layer over and contacting the first semiconductor layer;performing a third epitaxy to grow a second semiconductor layer over thefirst SiB layer, wherein the second semiconductor layer is free fromp-type impurities and n-type impurities; and recessing the isolationregions, so that the second semiconductor layer and a portion of thefirst SiB layer are higher than top surfaces of the isolation regions toform a semiconductor fin. In an embodiment, in the recessing theisolation regions, a first portion of the first semiconductor layer ishigher than the top surfaces of the isolation regions to form a portionof a semiconductor fin. In an embodiment, in the recessing the isolationregions, a second portion of the first semiconductor layer is lower thanthe top surfaces of the isolation regions. In an embodiment, the methodfurther comprises forming a gate stack overlapping a first portion ofthe first semiconductor layer; performing an etching process using thegate stack as a part of an etching mask to form a trench, wherein in theetching process, the second semiconductor layer is etched-through, and asurface of the first SiB layer is exposed to the trench; and forming asource/drain region in the trench, wherein the source/drain region is ofn-type. In an embodiment, the method further comprises performing afourth epitaxy to grow a second SiB layer over the second semiconductorlayer; and performing a fifth epitaxy to grow a third semiconductorlayer over the second SiB layer, wherein the third semiconductor layeris free from p-type impurities and n-type impurities.

In accordance with some embodiments of the present disclosure, asemiconductor device comprises isolation regions extending into asemiconductor substrate; a semiconductor fin between the isolationregions, wherein the semiconductor fin is higher than top surfaces ofthe isolation regions, and the semiconductor fin comprises a firstsemiconductor layer, the first semiconductor layer being free fromp-type impurities; and a first p-type semiconductor layer over andcontacting the first semiconductor layer; a gate stack on thesemiconductor fin; and a source/drain region extending into thesemiconductor fin, wherein the source/drain region contacts the firstp-type semiconductor layer, and the source/drain region is an n-typeregion. In an embodiment, the source/drain region comprises a bottomsurface contacting a top surface of the first p-type semiconductorlayer. In an embodiment, the source/drain region penetrates through thefirst p-type semiconductor layer, and the source/drain region contacts asidewall of the first p-type semiconductor layer. In an embodiment, thesemiconductor fin further comprises a second semiconductor layer overand contacting the first p-type semiconductor layer, the secondsemiconductor layer being free from p-type impurities and n-typeimpurities. In an embodiment, the semiconductor fin further comprises asecond p-type semiconductor layer over and contacting the secondsemiconductor layer; and a third semiconductor layer over and contactingthe second p-type semiconductor layer, the third semiconductor layerbeing free from p-type impurities and n-type impurities.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method of forming a semiconductor device, themethod comprising: depositing a first p-type semiconductor layer over aportion of a semiconductor substrate; depositing a first semiconductorlayer over the first p-type semiconductor layer, wherein the firstsemiconductor layer is free from p-type impurities; forming a gate stackdirectly over a first portion of the first semiconductor layer; etchinga second portion of the first semiconductor layer to form a trenchextending into the first semiconductor layer, wherein at least a surfaceof the first p-type semiconductor layer is exposed to the trench; andforming a source/drain region in the trench, wherein the source/drainregion is of n-type.
 2. The method of claim 1 further comprisingdepositing a second semiconductor layer over the portion of thesemiconductor substrate, wherein the second semiconductor layer isfurther free from p-type impurities, and the second semiconductor layeris underlying and in contact with the first p-type semiconductor layer.3. The method of claim 2, wherein in the etching, the first p-typesemiconductor layer is further etched-through, and a top surface of thesecond semiconductor layer is exposed to the trench.
 4. The method ofclaim 2, wherein a bottom surface of the trench is higher than a bottomsurface of the second semiconductor layer.
 5. The method of claim 1,wherein the etching stops on a top surface of the first p-typesemiconductor layer.
 6. The method of claim 1 further comprisingdepositing a second p-type semiconductor layer over the firstsemiconductor layer.
 7. The method of claim 6, further comprisingdepositing a second semiconductor layer over the second p-typesemiconductor layer, wherein the second semiconductor layer is free fromp-type impurities.
 8. The method of claim 7 further comprising forming agate electric over and contacting the second semiconductor layer.
 9. Themethod of claim 7 further comprising etching a dummy gate stack over thesecond semiconductor layer, wherein the second semiconductor layer isetched-through, and the etching stops on a top surface of the secondp-type semiconductor layer.
 10. The method of claim 1, wherein the firstp-type semiconductor layer is further free from n-type impurities.
 11. Amethod of forming a semiconductor device, the method comprising: formingisolation regions extending into a semiconductor substrate; etching toremove a portion of the semiconductor substrate between the isolationregions to form a trench; performing a first epitaxy to grow a firstsemiconductor layer in the trench, wherein the first semiconductor layeris free from p-type impurities and n-type impurities; performing asecond epitaxy to grow a first SiB layer over and contacting the firstsemiconductor layer; performing a third epitaxy to grow a secondsemiconductor layer over the first SiB layer, wherein the secondsemiconductor layer is free from p-type impurities and n-typeimpurities; and recessing the isolation regions, so that the secondsemiconductor layer and a portion of the first SiB layer are higher thantop surfaces of the isolation regions to form a semiconductor fin. 12.The method of claim 11, wherein in the recessing the isolation regions,a first portion of the first semiconductor layer is higher than the topsurfaces of the isolation regions to form a portion of the semiconductorfin.
 13. The method of claim 12, wherein in the recessing the isolationregions, a second portion of the first semiconductor layer is lower thanthe top surfaces of the isolation regions.
 14. The method of claim 11further comprising: forming a gate stack overlapping a first portion ofthe first semiconductor layer; performing an etching process using thegate stack as a part of an etching mask to form an additional trench,wherein in the etching process, the second semiconductor layer isetched-through, and a surface of the first SiB layer is exposed to theadditional trench; and forming a source/drain region in the additionaltrench, wherein the source/drain region is of n-type.
 15. The method ofclaim 14 further comprising: performing a fourth epitaxy to grow asecond SiB layer over the second semiconductor layer; and performing afifth epitaxy to grow a third semiconductor layer over the second SiBlayer, wherein the third semiconductor layer is free from p-typeimpurities and n-type impurities.
 16. A semiconductor device comprising:isolation regions extending into a semiconductor substrate; asemiconductor fin between the isolation regions, wherein thesemiconductor fin is higher than top surfaces of the isolation regions,and the semiconductor fin comprises: a first semiconductor layer, thefirst semiconductor layer being free from p-type impurities; and a firstp-type semiconductor layer over and contacting the first semiconductorlayer; a gate stack on the semiconductor fin; and a source/drain regionextending into the semiconductor fin, wherein the source/drain regioncontacts the first p-type semiconductor layer, and the source/drainregion is an n-type region.
 17. The semiconductor device of claim 16,wherein the source/drain region comprises a bottom surface contacting atop surface of the first p-type semiconductor layer.
 18. Thesemiconductor device of claim 16, wherein the source/drain regionpenetrates through the first p-type semiconductor layer, and thesource/drain region contacts a sidewall of the first p-typesemiconductor layer.
 19. The semiconductor device of claim 16, whereinthe semiconductor fin further comprises a second semiconductor layerover and contacting the first p-type semiconductor layer, the secondsemiconductor layer being free from p-type impurities and n-typeimpurities.
 20. The semiconductor device of claim 19, wherein thesemiconductor fin further comprises: a second p-type semiconductor layerover and contacting the second semiconductor layer; and a thirdsemiconductor layer over and contacting the second p-type semiconductorlayer, the third semiconductor layer being free from p-type impuritiesand n-type impurities.